The present invention relates to a process for the direct preparation of polyurethane powders from the components in an organic carrier phase.
Polyurethane powders and their technical use are known. The powders are generally prepared by a difficult process of grinding the corresponding granulate. The direct synthesis of the powders from their components in an organic carrier phase by the method of suspension polymerization is a simpler process, with the best results being obtained when the carrier phase consists of aliphatic and/or alicyclic hydrocarbons. Interface active auxiliary substances are essential for carrying out this direct synthesis. The auxiliary substances have the function of converting the reaction components into finely divided emulsions and keeping the resulting polyurethane in emulsion or suspension until the end of the reaction.
In German Auslegeschrift 2,816,170, special polyoxyalkylene-polydimethylsiloxane graft or block copolymers are proposed as interface active auxiliary substances for the direct synthesis of polyurethane powders from aliphatic hydrocarbons in a carrier phase. Polymers containing polydimethylsiloxane are, however, generally not suitable for polyurethane applications since they tend to produce surface defects in the molded products and give rise to difficulties in lacquering. Extreme care must be taken to remove these auxiliary substances as far as possible quantitatively from the powder. In German Auslegeschrift 2,456,927, block and graft copolymers of polylactones and long chain alkyl esters of (meth)acrylic acid and corresponding products obtained from polyoxyalkylene glycols and long chain alkyl esters of (meth)acrylic acid are described as interface active auxiliary substances for the direct preparation of polyurethane powders by the method of suspension polymerization in an aliphatic carrier phase of hydrocarbons. These auxiliary substances have the disadvantage of being OH functional and therefore incorporated in the polyurethane produced so that changes in the mechanical properties of the products may occur and the auxiliary substances cannot be reused.
In German Auslegeschriften 2,556,945, 2,559,769 and 2,442,085 and in U.S. Pat. Nos. 4,032,516 and 3,787,525, copolymers of N-vinylpyrrolidone and long chain .alpha.-olefins are used as interface active auxiliary substances for the direct synthesis of polyurethane powders in a hydrocarbon carrier phase.
These auxiliary substances cannot be built into the end product but are difficult to desorb from the surface of the powders produced, with the result that elaborate washing operations are necessary for recovering the auxiliary substances.
It was an object of the present invention to provide an improved process for the direct preparation of polyurethane powders which would result in finely divided polyurethane dispersions and in which the emulsifiers could easily be removed after formation of the powders.